Mochi - Write notes and flashcards with Markdown and study them with spaced repetition.

CleverDeck - Learn languages more efficiently with spaced repetition

Yak Tack - remember. words.

ACT Flashcards from Magoosh - Everything you should have learned in high school

Zorbi - Spaced-Repetition Flashcards - Other tools are built by boomers, Zorbi is built by students.

Anki - Anki is a program which makes remembering things easy. Because it's a lot more efficient than traditional study methods, you can either greatly decrease your time spent studying, or greatly increase the amount you learn.